1. Field of the Invention
The present invention relates to a focus adjustment apparatus that performs focusing in an imaging apparatus such as a digital still camera, a video camera or a film-based camera.
2. Description of the Related Art
As a general system that uses a light flux passed through a photographic lens in a focus detection and adjustment method of a camera, there are a contrast detection system and a phase difference detection system.
The contrast detection system is frequently used in the video camera or the digital still camera, and an image sensor is used as a focus detection sensor. This system determines, by focusing on an output signal of the image sensor, particularly information of a high-frequency component (contrast information), a position of the photographic lens where its evaluation value is largest as an in-focus position.
However, as it is referred to as a hill-climbing method, the system must acquire an evaluation value by slightly moving a focus position of the photographic lens, and move the focus position until the evaluation value is found to be largest. Hence, the system is not suited to a high-speed focus detection operation.
Focus detection of the phase difference detection system is a technology frequently used in a single-lens reflex camera, which has contributed to practical use of a single-lens reflex camera with automatic focus (AF) detection. For example, in a digital single-lens reflex camera, a focus detection unit that includes a secondary imaging optical system performs AF of the phase difference detection system.
The focus detection unit includes a pupil dividing unit that divides a light flux passed through an exit pupil of the photographic lens into two areas. The light flux divided into the two areas forms, via an optical path division optical system located in a mirror box, an image on a set of focus detection sensors by the secondary imaging optical system. Then, by detecting a shifting amount of a signal output according to light reception amounts of the sensors, namely, a relative positional shifting amount in a pupil dividing direction, a shifting amount in a focus direction of the photographic lens is directly acquired.
Thus, a defocusing amount and a defocusing direction are simultaneously acquired through a storage operation by the focus detection sensor. This enables a high-speed focus adjustment operation. During imaging after the focus detection, the optical path division optical system is retracted outside the imaging light flux, and the image sensor is exposed to acquire a captured image.
There is a technology for achieving high-speed AF even during electronic viewfinder observation or moving image capturing where an AF function of the phase difference detection system is provided to the image sensor, and a display unit such as a backside liquid crystal checks an image in real time. For example, there has been developed a technology for providing, in a certain light receiving element (pixels) of the image sensor, a pupil dividing function by setting a sensitivity area of a light reception unit eccentric from an optical axis of an on-chip microlens.
AF of the phase difference detection system is performed by using the pixels as focus detection pixels and arranging the pixels at a predetermined interval in a group of imaging pixels. Arranging places of the focus detection pixels correspond to defective portions of the imaging pixels, and hence surrounding imaging pixel information is interpolated to generate image information. In this example, AF of the phase difference detection system can be executed on an imaging plane. Thus, high-speed and highly accurate focus detection can be performed even during electronic viewfinder observation or moving image capturing.
During the electronic viewfinder observation or the moving image capturing, an amount of light reaching the image sensor is adjusted or a blur amount of a captured image is adjusted according to a user's image forming intension. This may necessitate adjustment of a diaphragm aperture diameter of the photographic lens. It is desired that an image that is always placed in in-focus state by focus adjustment is captured even in such a situation. However, since focus detection and adjustment are executed by the light flux passed through the photographic lens, the above-mentioned focus detection and adjustment method is affected in no small part by a change in diaphragm aperture diameter of the photographic lens.
To deal with this problem, Japanese Patent Application Laid-Open No. 7-111614 discusses a technology for inhibiting a focus adjustment operation when a diaphragm aperture diameter is adjusted in focus adjustment of the contrast detection system. This technology can prevent an erroneous focus detection operation when the diaphragm aperture diameter is adjusted.
Japanese Patent Application Laid-Open No. 03-214133 discusses a technology for correcting, in focus detection of the phase difference detection system, when a diaphragm of the photographic lens blocks (vignettes) a light flux used for the focus detection, an output signal in view of an amount of light that has not reached the focus detection unit due to the blocked light flux. This technology enables highly accurate focus detection by correcting the output signal according to a vignetting state of the light flux used for the focus detection even when the diaphragm aperture diameter is adjusted
However, in the technology discussed in Japanese Patent Application Laid-Open No. 7-111614, the inhibition of the focus adjustment operation creates a possibility that when an object moves, the system will not be able to follow the movement, resulting in capturing of an out-of focus image. The out-of focus image may be captured even while a focus adjustment operation is executed again after a predetermined period of time. In both cases, there is a possibility that an image felt unnatural during observation may be recorded as a moving image.
In the technology discussed in Japanese Patent Application Laid-Open No. 03-214133, the correction of the light amount of the focus detection signal in view of vignetting of the photographic lens is performed based on design information of the photographic lens side. However, a degree of vignetting is determined by several frame members including a diaphragm aperture of the photographic lens, and each component has a manufacturing error in external shape or arrangement. Hence, even when the amount of light is corrected based on only the design information of the photographic lens, there is a possibility that an error may occur in focus detection result.
In other words, during the moving image capturing, when a change in diaphragm aperture diameter is accompanied by a change in degree of vignetting, a focus detection result may vary between before and after the change in diaphragm aperture diameter due to the error. Nevertheless, when the photographic lens is driven according to the acquired focus detection result including the error, discontinuous points may be generated in a focus adjustment state of an image being captured, creating a possibility that an image felt unnatural during observation may be recorded as a moving image.